This invention relates to a low loss ultra broadband traveling wave divider/combiner. More particularly, the present invention relates to forming a low loss ultra broadband traveling wave divider/combiner on an integrated circuit.
Traveling wave power dividers/combiners (TWD/Cs) have been used for dividing high frequency input signals (such as RF or microwave signals) into two or more divided signals, where each divided signal is amplified by a separate amplifier, and the amplified divided signals are combined. TWD/Cs are known to divide and combine signals to/from 3 to 6 power amplifiers. TWD/Cs are commonly used because they are compact, have low loss and are viable well into the millimeter wave frequency range.
A typical TWD/C which is used in the microwave band is known as the so-called Wilkinson type power divider/combiner circuit. FIG. 6 illustrates a conventional Wilkinson divider combiner. As shown in FIG. 6, an input 41 is connected to the end of a first transmission line 42 and to the end of a resistor 43. Another input 44 is connected to the end of another transmission line 45 and to the other end of the resistor 43. The other ends of the transmission lines 42 and 45 are connected to each other at a combining node 46, which is connected to output 47. The resistor 43 is an isolation resistor which absorbs a reflective signal between nodes 41 and 44, thus attenuating an unbalance between the nodes 41 and 44. The length of each of the transmission lines 42 and 45 are typically set at about xc2xc of the wavelength of the input signal. TWD/Cs are typically designed using a cascade of Wilkinson power dividers, each having a different power division ratio. For example, a 4-way TWD/C may use the power division ratios of 4:1, 3:1 and 2:1. Since most systems require power amplifiers with a high power added efficiency, the combiner must have low loss and be well matched to the designed input and output impedance of the amplifier, which is typically 50 ohms. However, the typical TWD/Cs have a limited active bandwidth.
It is known to improve the isolation characteristics of the transmission lines by placing a capacitor in series with the isolation resistor. This approach is described in U.S. Pat. No. 5,977,843 to Watanabe, entitled xe2x80x9cHigh Frequency Power Divider And High Frequency Power Combiner.xe2x80x9d Watanabe places the capacitor in series with the resistor to cancel the parasitic inductive properties of the signal line connecting the isolation resistor, which Watanabe describes as parasitic transmission lines. Watanabe discloses that the parasitic inductance causes the isolation characteristics to deteriorate and thus the capacitor prevents the deterioration of the isolation characteristics.
It is also known in the art to produce low loss microstrip lines for monolithic microwave integrated circuits. One approach is described in U.S. Pat. No. 5,753,968 to Bahl et al. entitled xe2x80x9cLow Loss Ridged Microstrip Line for Monolithic Microwave Integrated Circuit (MMIC) Applications, herein incorporated by reference. Bahl et al. discloses to form a microstrip line by forming a strip conductor on top of a dielectric which is formed on a substrate. The dielectric preferably has a dielectric constant less than the dielectric constant of the substrate. In this manner, the dielectric reduces the dissipation loss of the microstrip line.
However, the prior art typically only provides a TWD/C which is suitable over a relatively small bandwidth. Accordingly, there is need for a TWD/C which is effective over a large bandwidth. Additionally, there is a need for a TWD/C which provides a broadband multiple division/combination which can be fabricated as an integrated circuit.
The present invention may comprise an ultra broadband traveling wave divider/combiner comprising; a substrate; at least one divider/combiner unit formed on said substrate, each divider/combiner unit of said at least one divider/combiner unit comprising: a first transmission line which receives a portion of an input; a second transmission line which receives the remaining portion of said input; a first resistor connected in series with a first capacitor across outputs of said first transmission line and said second transmission line; a third transmission line connected to the output of said first transmission line and which receives an input from said first transmission line; a fourth transmission line connected to the output of said second transmission line and which receives an input from said second transmission line; a second resistor connected in series with a second capacitor across outputs of said third transmission line and said fourth transmission line; an output connected to the output of said fourth transmission line.
In a further aspect of the present invention, any of the transmission lines of the present invention may further comprise a dielectric layer formed on said substrate; and a metal layer formed on said dielectric layer, wherein said metal layer is connected to one of said resistor and said capacitor.
In a further aspect of the present invention, the metal layer in the transmission lines of the present invention may overhang said dielectric layer such that a via hole is formed, and the metal layer may connect to the resistor and/or the capacitor through first via hole.
The present invention may comprise an impedance step down unit; a capacitor connected to the impedance step down unit; and a plurality of divider/combiner units cascaded.
In another aspect of the invention the impedance step down unit may comprise a first transmission line and a second transmission line connected to the output of the first transmission line.
In a further aspect of the invention the capacitor may be connected between the output of the first transmission line and ground.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention. The invention itself can better be understood by reference to the following detailed description and the accompanying drawings.